Piezoceramic multilayer actuators, which are also referred to as multilayer stacks, are stacked layer structures, having alternating layers of piezoceramic and internal electrode material. In this structure the internal electrodes reach to the side faces of the multilayer actuator, to achieve the biggest possible active volume in the piezoceramic layers per structural volume of the multilayer actuator.
However if the internal electrodes reach to the outsides of the multilayer actuator, multilayer actuators with such a structure cannot be operated directly in electrically conducting fluids or gases. These fluids or gases would result in an electrical short circuit of the different internal electrodes and thus to failure of the multilayer actuator. This is specifically disadvantageous when the piezoceramic multilayer actuators are used in fuels of internal combustion engines, as for example in direct injection systems in the automotive field. These fuels generally contain certain proportions of water or additives, which would produce an electrical short circuit between internal electrodes projecting to the outside. The same disadvantage results with use in an environment with high air humidity, for example when the multilayer actuator is used in pneumatic valves or as a mass flow controller. Electrical passivation or insulation of the piezoceramic multilayer actuator from the medium surrounding it is therefore essential.
However the material for passivating the multilayer actuator has to have a similar extension capacity to the multilayer actuator itself. The achievable homogeneous extension of the multilayer actuator is up to 0.2% of its length. However if poling cracks occur in certain regions in the piezoceramic multilayer actuator, for example in inactive contacting zones, considerably greater relative extensions of the multilayer actuator are achieved in these regions compared with its homogenous extension. Therefore a coating material used to passivate the multilayer actuator would generally have to have a cyclical mechanical long-term extensibility in the order of magnitude of more than 10%. If such a passivation is utilized for example in a direct injection system in the automotive field, the material also has to have temperature stability up to approximately 180° C. Only silicone elastomers satisfy these material requirements but they are insufficiently resistant in fuels and demonstrate high steam permeability.
One solution deployed in the prior art to protect the piezoceramic multilayer actuator utilizes encapsulation of said piezoceramic multilayer actuator from its environment. This encapsulation consists of a hermetically sealed metallic housing, which does not impede the useful extension of the piezoceramic multilayer actuator. Such an encapsulation is also referred to as an extension pipe or corrugated pipe. These encapsulation solutions are however costly and it has also not been able to realize them to date for higher ambient pressures.